1. Field of the Invention
The present invention relates to a revolution control apparatus for controlling motor revolution speed.
2. Description of the Prior Art
Conventionally, two methods are proposed for controlling motor revolution speed as follows. First, a tachometer generator system is proposed wherein a tachometer generator is mounted on a rotating shaft of a motor and an output voltage obtained in proportion to the revolution speed of the motor is set so as to be constant. Second, a revolution pulse system is proposed wherein a slit disc is mounted on a rotating shaft of a motor, and an output pulse having a frequency proportional to the revolution speed of the motor is obtained by a photosensor, a magnetic head or the like so as to keep the pulse period constant. Among these systems, in order to accurately set the revolution speed of the motor and minimize a variation in revolution speed, the revolution pulse system is generally superior to the tachometer generator system. This is because the tachometer generator has low detection precision and a low response to the revolution speed, and is affected by changes in temperature. However, the pulse generator has high detection precision and may not be affected by changes in temperature, thereby providing highly precise motor revolution speed detection. Therefore, the pulse generator is used when a high detection precision is required, whereas the tachometer generator which has a simple construction and results in low cost is used when a low detection precision is needed. Even though the pulse generator provides high detection precision, it has the following drawback.
When the number of slits formed in the slit disc is increased, the number of pulses obtained upon one revolution of the motor is proportionally increased. In principle, revolution speed correction of the motor is performed in proportion to the number of pulses. In this sense, it might be expected that the revolution speed precision would be greatly improved. However, in fact, an interval error occurs between the adjacent slits at the time of manufacture of the slit disc if the number of slits is increased. As a result, the revolution speed precision is generally degraded.
The relationship between the slits and the pulses will be described with reference to the slit discs and output pulses shown in FIGS. 1(a) to 1(d). The slit disc shown in FIG. 1(a) has a single slit. A time interval from a given pulse to the next pulse is always constant. When the revolution speed of the motor is controlled to obtain a predetermined time interval between two adjacent pulses, the motor revolution speed is kept constant. However, it should be noted that speed correction is performed only once during one revolution of the motor. As a result, a speed variation (i.e., jitter) during one revolution may not be compensated for. The slit disc shown in FIG. 1(b) has eight slits. The slits are ideally formed in the disc at equal angular intervals, no interval error is allowed between any two adjacent slits. In this case, error correction is performed 8 times per revolution, so that the jitter described above can be also corrected. However, it is impossible to completely eliminate the interval error between any two adjacent slits because of a lack of precision in the process for manufacturing the slits. In fact, interval errors occur as shown in FIG. 1(c). If the motor revolution speed is corrected with such a disc so as to keep the pulse period constant, it is readily understood that a variation in revolution speed is thus increased. The slit disc shown in FIG. 1(d) has an extreme interval error. Here assume that a Hall motor is used. In general, the Hall motor can be readily driven to increase the revolution speed thereof but is difficult to stop. The revolution speed of the Hall motor is naturally decreased by air resistance or friction loss. In this manner, the Hall motor has a fast response time for increasing the speed, but a very slow response time for decreasing the speed. If the slit disc shown in FIG. 1(d) is used to control the Hall motor, the number of revolutions of the motor is increased at a wide pulse interval and is decreased at a narrow pulse interval. As a result, the average motor speed is substantially determined by the wide pulse intervals. Even if the oscillation frequency precision of a quartz oscillator for measuring the pulse intervals were to be greatly improved, the average motor speed would nevertheless undergo an error corresponding to an unequal interval between two adjacent slits. The average motor speed is thus affected by the precision of the slits, resulting in inconvenience.